The present invention relates to a zoom lens useful in photography. More particularly, the present invention relates to a wide-angle zoom lens for single lens reflex cameras that is capable of providing a view angle of about 80.degree. at the wide-angle position while permitting a very effective compensation of aberrations, particularly distortion.
The performance of a zoom lens varies considerably during zooming and minimization of this variation has long been one of the major targets to be attained in system design. Most wide-angle zoom lenses employ the "two-lens-group" system and basically consist of a retrofocus type configuration. It is very difficult to perform effective compensation of barrel distortion at the wide-angle position of the retrofocus type lens system. This is particularly true with a wide-angle zoom lens of the type contemplated by the present invention which provides a viewing angle of about 80.degree., and it is generally understood that the manufacturer must bear with a certain amount of residual distortion in order to achieve a compromise between the need for system miniaturization and the minimization of the residual amounts of other aberrations.
The problems of the conventional zoom lens are hereunder described by reference to the two-lens-group type system. As is well known, zooming with the conventional two-lens-group type system from the short focal length to the long focal length is accomplished by varying, specifically stated, reducing, the distance between the front and rear groups. The diaphragm is customarily positioned either within or just in front of the rear group and is displaced simultaneously with the rear group.
The lenses responsible for compensation of barrel distortion that may develop in the system are a positive lens positioned closer to the object than the diaphragm and a negative lens positioned closer to the image than the diaphragm. Greater compensation effects are attained as the two lenses are positioned farther away from the diaphragm, namely as the primary beam of light intersects these lenses at positions farther away from the optical axis.
This fact should be taken into consideration in the following discussion of distortion that has occurred in the conventional two-lens-group type zoom lens at the short focal length side. Any barrel distortion that may develop at the short focal length side results from the fact that the front group has a negative refractive power and is distant from the rear group, i.e., far from the diaphragm. This suggests the possibility of reducing the distortion by decreasing the refractive power of the front group, but then, the size of the overall system is increased. Therefore, in order to compensate for barrel distortion without increasing the overall size of the system, it has been customary design practice to depend on a positive lens positioned within the front group and a negative lens positioned in the rear group closer to the image than the diaphragm.
In order to achieve satisfactory compensation of the distortion by this approach, only two methods are available; one method is to position the positive lens (in the front group) or the negative lens (in the rear group) farther away from the diaphragm, and the other method depends on increasing the refractive power of either one of the lenses. However, the first method is unable to satisfy the need for a compact system because not only the diameter of the front or rear lens but also the overall length of the system is increased. The second method is also undesirable since if the refractive power of the positive lens in the front group is increased, the refractive powers of negative lenses in the front group must also be increased in order to maintain the same negative refractive power of the front group. This however causes increased distortions of high orders, and as a result, the variation in the distortion with respect to a change in the angle of incident light is increased, particularly in the region of large incident angles, or residual coma aberration of high order occurs, making it difficult for the system to maintain the desired performance.
One may try to increase the refractive power of a negative lens positioned in the rear group closer to the image than the diaphragm, but this method has the same problems as described in the previous paragraph since the rear group as a whole has a positive refractive power.
It will be understood from the discussion above that so long as the conventional two-lens-group type zoom lens system is employed, one must bear with the existence of a certain amount of residual barrel distortion at the short focal length side to strike a balance with other aberrations and to realize a compact overall system.
Several attempts at effective compensation of the distortion have of course been reported. For example, Japanese Patent Publication No. 13003/1984 discloses a system that employs aspherical lens surfaces for the purpose of reducing the distortion to a little more than -3% with a viewing angle of 84.degree.. However, this sytem has the general disadvantage of high manufacture cost resulting from the use of non-spherical lens surfaces.
Japanese Patent Publication No. 13850/1982 proposes a modified two-lens-group type zoom lens system wherein the rear group is divided into two subgroups, one being positive and the other negative. This system permits the three groups to be moved independently and thereby provides a greater degree of freedom in zooming operation. At the same time, this system depends on the negative lens subgroup for compensation of the distortion so that its amount and the variation during zooming can be minimized. However, this system permits a viewing angle of as small as 48.degree. at the wide-angle position and is obviously unsuitable in designing a zoom lens system of the type contemplated by the present invention which is capable of providing a viewing angle of as wide as 80.degree..